Why do many references recommend placing tuckpointing mortar in 1/4-inch layers? It would seem possible to place a filled mortar joint with one pass. What is the advantage of using multiple passes?

Placing mortar in 1/4-inch lifts, as recommended by the Brick Industry Association in Technical Note 7F and by other authorities, will help ensure a tightly filled joint. Each layer is applied after the previous layer has become thumbprint hard. This allows mortar to be pressed tightly into the joint. As a result, the mortar will exert pressure against the upper and lower bond surfaces to ensure a better compacted joint and to potentially improve mortar bond over joints placed in one pass. In many cases, repointing is needed to reduce the rate of water penetration. Typically, removal of the existing mortar is the most expensive part of the operation. With this in mind, the cost savings associated with placing mortar in one lift would not seem to be justified.